1. Field of Invention
This invention relates to an electric fence charger and more particularly to an improved DC-DC voltage converter that maintains a relatively constant voltage output within a specified battery operating voltage range and halts operating upon discharge of the DC battery below a specified battery discharge reference voltage.
2. Description of the Prior Art
As is well known, electric fence chargers have been used for many years to impress shocking potentials on wire fences to train cattle or farm animals generally from straying beyond predetermined boundaries. Electric fence chargers provide electrical pulses to the fence wire that are spaced apart by either a constant or variable time and which inflict electric shocks to an animal contacting the wire.
Electric fence chargers are designed to operate under a number of constraints to minimize the risk of electrocution and to continue to operate under severe weather and temperature conditions. In addition, fence chargers are preferably designed to operate at power levels too low to pose the risk of starting a fire in dry wood or plants. And since the fence chargers are typically powered by a low voltage, rechargeable storage battery, it is important that the fence charger operate the charger fence intermittently to provide the narrow pulse width pulses every second or so.
Early fence chargers were constructed with a mechanical oscillator, wherein an induction coil is charged and discharged by spring loaded oscillating switch. Such chargers in practice proved costly by reason of inordinate drains on the power supply and failures through shorting of the associated fence wire or because of arcing between the contacts of the oscillating switch. More recently, electronic circuit fence chargers have been developed which substitute electronic oscillators for mechanical oscillators that periodically switch current through output step-up transformers to provide the requisite high voltage potential to be discharged through the fence wire. Such electric fence chargers are shown, for example, in U.S. Pat. No. 3,772,529.
Usually, currently available fence chargers apply narrow width, high voltage, low current electric pulses generated through a DC-DC converter and electronic timing circuits associated therewith from a low voltage, rechargeable battery, such as a deep-discharge, 12 volt, lead acid vehicle or boat battery. The 12 volt batteries are typically recharged periodically after they are discharged as current is drawn by the electric fence charging circuit.
The fence wire to be charged normally presents a very high resistance, capacitive load, that is proportional to the length of the wire, when the fence is "unloaded". The average fence may have a capacitance between 0.015 microfarads to as much as 0.1 microfarads or even higher depending on the length of the fence wire. If the fence wire is "loaded" by water, ice, moist weeds or the like, or if an animal body contacts the wire, or the wire is on the ground, a resistance on the order of less than a thousand ohms may be presented to the output of the fence charger. The load presented may affect the current drawn by the charger and draw down the battery voltage to a point where the battery may be permanently damaged.
Apart from the accidental excess discharge of the battery, the inefficiency of the normally operating voltage converter circuit may cause the battery energy to become depleted to a damaging level before attention is given to recharging the battery.
Accordingly, a need exists for a reliable, energy efficient electric fence charger which avoids excess discharge and damage to the battery.
A need also exists for a DC-DC converter circuit that insures that the electric fence shock pulse voltage remains relatively constant in the operating range of the battery between full charge and a discharge reference voltage.
A further need exists for safely discharging any residual high voltage on the high voltage capacitor after the battery voltage falls to the discharge reference voltage.